Fiber optic cables have established themselves as the transmission medium of choice for high-speed communications. Silica fibers can transmit optical signals within the 1.3 to 1.5 .mu.m band over long distances with small distortion. The width of this band corresponds to a bandwidth of 3.times.10.sup.14 Hz or 300 terahertz.
This huge bandwidth remains unexploited, in part, because both the transmitter and receiver are based on electronic components. For instance, the receiver includes a photo-detector which detects the intensity of the light signal and converts it to an electrical signal. However, conventional electronics are considerably slower than 300 terahertz. Conventional silicon semiconductor electronics have an upper limit of about 1 gigahertz (1.times.10.sup.9 Hz), although this limit is being gradually pushed up somewhat.
Compound semiconductors are known to offer higher speed performance. In particular, InP and its lattice-matched alloy InGaAs show particular promise for high-speed operation in this region of the infrared at which silica fibers transmit.
At these gigahertz frequencies, circuit design with discrete components becomes difficult because of parasitics and other reasons. As a result, much of the InP work has concentrated on integrated opto-electronic receivers. That is, an InP integrated circuit is built which includes both an InGaAs photo-detector and InP-based electronics, such as an amplifier. On an integrated circuit chip, parasitic capacitance and other delays can be minimized so that the speed of the circuit begins to approach that of the individual components. Receiver speeds of about 3 GHz have been reported for such circuits by Chang et al. in "A 3 GHz Transimpedance OEIC Receiver for 1.3-1.5 .mu.m Fiber-Optic Systems," IEEE Photonics Technology Letters, volume 2, 1990, pages 197-199.
Nonetheless, even 3 GHz represents a very small fraction of the potential bandwidth of fibers. In order to avoid the limitations of electronics, a favored architecture for a fiber optic communication network uses wavelength-division multiplexing (WDM). A simplified version of WDM uses N lasers having N different oscillation frequencies within the chosen transmission band of the fiber. Separate electronic data signals modulate each of the lasers so that the N data signals are carried on N separate optical carrier frequencies. By optical means, the N optical signals are combined onto the one optical fiber. At the receiving end, the signals are optically separated by some kind of optical filter or splitter before being detected in N photo-detectors. The optical channels can be separated by as little as 1 nm. The photo-detectors need only have speeds corresponding to the individual data signals, e.g., 3 GHz, while the capacity of the receiver is increased by the factor N.
Lee et al. have reported an integrated four-channel receiver in "4-channel InGaAs/InP transimpedance optical receiver array OEICs for HDWDM applications," Proceedings of European Conference on Optical Communications, 1990, pages 195-1 through 195-4. They discussed a receiver in which the WDM optical signal on a fiber is focused onto an optical grating, which diffracts the different carrier frequencies to different positions. Each position corresponds to the photo-detector of one of four optical receivers. Each receiver uses a PIN diode for its photo-detector and has its own pre-amplifier section for the output of the photo-detector. Such an integrated multi-channel optical receiver has obvious advantages of economical fabrication and ease of alignment. Chang et al. have reported an integrated dual MSM detector in "High performance monolithic dual-MSM photo-detector for long-wavelength coherent receivers," Electronics Letters, volume 25, 1989, pages 1021-1022.
In one class of proposals for a WDM fiber-optic communications network, a receiving station would receive multiple WDM channels but would electrically output only a selected one of these. A possible example of such a receiving station receives different cable television services from different vendors on different WDM channels but outputs to the television input only one signal representing one vendor's services. Other examples exist in broadcast architectures for WDM telephone networks in which the receiver at the customer's premise can be used to select the wavelength-based channels. The Lee et al. multi-channel receiver is disadvantageous for such a switched optical receiver because it replicates the pre-amplifier N times for an N-channel receiver. Each pre-amplifier in the Lee et al. design includes 21 electronic components in addition to the attached optical detector. They stated that their 4-channel receiver represented the then highest level of InP opto-electronic integration. The N-times replication of components severely impacts yield of an advanced design, which is difficult enough to fabricate. Although Lee et al. did not suggest turning their PIN optical detectors on and off, it would be difficult to do so. A PIN photo-detector will output an electrical signal whenever it is illuminated with light, regardless of the reverse bias condition.
Yamanaka et al. have disclosed a four-channel, switchable receiver in "A 1.5 Gbit/s GaAs four-channel selector LSI with monolithically integrated newly structured GaAs ohmic contact MSM photodetector and laser driver," IEEE Photonics Technology Letters, volume 1, 1989, pages 310-312. Similarly to Lee et al., each of their MSM photo-detectors had a dedicated amplifier. Switching was performed on the amplified photo-currents. As a result, their GaAs integrated circuit had 369 components.